A drill bit at the end of a bottom hole assembly (BHA) is used to drill a hole through earth formations. The drill bit has a design with a cutting structure to accomplish this task. Models of the cutting structure can predict performance in terms of rate of penetration (ROP), force, torque, side force, vibration, walk tendencies, steerability etc. A drill bit may contain a secondary cutting structure that is intended to further enlarge the hole, such as a bi-center bit. For purposes of this disclosure, these secondary cutting structures may be considered to be part of the drill bit and not part of a reamer.
A reamer is utilized to enlarge a borehole through earth formations. The reamer has a design with a cutting structure to accomplish this task. Models of the cutting structure can predict performance in terms of rate of penetration (ROP), force, torque, side force, vibration, walk tendencies, steerability, etc.
A reamer may exist at the end of a BHA (without a drill bit) if the pilot hole formed by a drill bit already exists. Typically a reamer is utilized above a drill bit in the same BHA. Multiple reamers can also be deployed, each enlarging a different increment of hole size (with our without a drill bit). Multiple reamers of the same enlargement increment might also be used for redundancy in case of a failure of one cutting structure. Reaming can occur both in the downward and upward directions along the borehole.
A reamer may employ a fixed cutting structure, such as a single piece hole opener, or an expandable/retractable cutting structure for passing through restrictions in the wellbore completion, or to enlarge only specific sections of a borehole for specific purposes. Selective control of an expandable/retractable reamer could also be used to keep a reamer dormant as a backup in case of failure of a primary reamer cutting structure.
A simple way to characterize the performance of the drill bit and reamer cutting structures is needed. With a simple characterization, the performance of these cutting structures can be easily compared across a range of lithology and drilling parameters and evaluated against a set of constraints.